The present invention is related to an image reproduction system wherein a developed image is transferred from an image-forming member to a receptor material via at least one intermediate transfer member with control of image distortion.
In a typical image reproduction system such as a printing or a copying system, a latent image is formed on an image-forming member by image-wise exposure using a known graphical process. The image-forming member can be an endless member such as a drum or a belt. Typical graphical processes include amongst others magnetography, ionography, elcography and electrography, particularly electrophotography. At present electrophotography is the most widespread. In the latter process, a charged latent image is formed on a pre-charged photosensitive member by image-wise exposure to light. The latent image is subsequently made visible on the image-forming member with charged toner at a development zone. After the development of the latent image, the developed toner image is transferred directly to a receptor material. The receptor material can be in the form of a web or in sheet form. In the latter case, the receptor material is preferably on a conveyor. An example of such an image reproduction system is disclosed in European patent EP629924 (Xeikon N.V.). A disadvantage of these direct transfer type of image reproduction systems are the stringent requirements which the recording media have to meet. It is widely known that for instance the electrical and thermal properties of the receptor material and particularly the accurate control of these properties determine to a large extent the quality and reproducibility of the images which are transferred and potentially fixed to the receptor material. The control of these properties, i.e. the conditioning of the medium, can be implemented in various ways, such as e.g. demonstrated in EP629925 (Xeikon N.V.). In general however, to enable the ability to print on a wide range of recording media one has to go first through an elaborate medium qualification procedure and thereafter through a demanding medium condition procedure.
Amongst others, in order to overcome or at least facilitate these procedures, reproduction systems of the new generation are provided with at least one intermediate transfer member between the image-forming member and the receptor material. In such systems the developed image is transferred from the image-forming member to the receptor material via one or more intermediate transfer members, usually in the form of endless belts or drums. A typical example of such a system is disclosed in U.S. Pat. No. 6,047,156 (De Bock et al., Xeikon N.V.). It is a clear benefit that the use of intermediate transfer members obviates the need for the conditioning of the receptor material or at least makes the conditioning less demanding. The use of intermediate transfer members introduces extra image transfer zones, i.e. regions where a first moving image carrying member and a second moving image carrying member contact each other in order to transfer a developed image from the first moving image carrying member to the second moving image carrying member. For instance, the reproduction system as disclosed in U.S. Pat. No. 6,047,156 uses two intermediate transfer members per side and by consequence each developed image has to pass through three transfer zones, being the transfer zones between the image forming member and the first intermediate transfer member (i.e. the primary belt), between the first and the second intermediate transfer member, and between the second intermediate transfer member and the receptor material. One of the problems which may appear at each transfer zone is image distortion and particularly image stretching or shrinking. The term xe2x80x9cimage distortionxe2x80x9d as used herein is intended to include both image size reduction as well as image size magnification. There are a number of parameters which may affect image magnification such as the contact pressure, the temperature and the properties of the respective image-carrying members, such as for instance surface roughness, thickness, elasticity, stiffness and surface energy. But even when all these parameters are properly controlled, the mutual forces in the contact zones exerted by the respective moving image-carrying members on each image carrying member will significantly influence image magnification. These forces are to a large extent determined by the drive and coupling strategy of the image carrying members in the image reproduction system.
It is an object of the invention to provide an image reproduction system having in operation at least one intermediate image carrying member contacting at least one other image carrying member, wherein these respective image carrying members are driven in a smooth and slipless way in order to control image distortion and particularly image stretching and shrinkage in the transfer contact zone.
It is a further object of the invention to provide an image reproduction system having at least one intermediate image carrying member, wherein during start-up the respective image carrying members are driven and coupled in a smooth and slipless way to control and limit the overall image distortion and particularly image stretching and shrinkage.
In an aspect of the invention a method is disclosed for controlling image distortion in the transfer contact zone between a first and a second moving image-carrying member, being part of an image reproduction system, by driving said first image-carrying member with a first drive device, capable of speed and torque control, and driving said second image-carrying member with a second drive device, capable of speed and torque control, such that the force exerted by the first moving image-carrying member on the second moving image-carrying member is countered by the force exerted by the second moving image-carrying member on the first moving image-carrying member.
The countering of the force exerted by the first moving image-carrying member on the second moving image-carrying member by the force exerted by the second moving image-carrying member on the first moving image-carrying member is preferably such as to result in a substantially balanced condition. This balancing is preferably achieved by substantially equally dividing the additional load created in the transfer contact zone by engaging said first moving image-carrying member against said second moving image carrying member over said first and said second motor. At start-up, the first and second image-carrying members are disengaged. The first image-carrying member is driven in speed control mode by a first drive device, capable of speed and torque control. The second image-carrying member is driven in speed control mode by a second drive device, capable of speed and torque control. Both image-carrying members are ramped up to about the same predetermined linear speed. As, when coupled, the first image-carrying member will be the xe2x80x9cslavexe2x80x9d and the second image carrying member will be the xe2x80x9cmasterxe2x80x9d, it may be advantageous to drive the first image-carrying member at a slightly higher speed, typically up to 5% higher, compared to the speed of the second image-carrying member. The de-coupled current and voltage values of the drive devices are stored. Then the maximum current of the first drive device is set to a value slightly higher than its de-coupled value. Next, the two moving image-carrying members are coupled thereby creating a transfer contact zone. In a preferred construction, the first and the second image-carrying member pass over respective guide rollers so positioned, in the coupled position of the first image-carrying belt with the second image-carrying belt, to form a transfer contact zone therebetween. At least one of these guide rollers is movable to enable the first and the second image-carrying belt to be de-coupled from each other.
Due to the coupling action, the first drive device goes into a torque controlled mode and its current equals the set point current. The second drive device is still speed controlled. By consequence, due to the losses created by the coupling in the transfer zone, i.e. the additional load, the current of the second drive device increases. This is now a clearly unbalanced situation as the losses are fully compensated by the second drive device. An approach to obtain a balanced situation is as follows. The set point current of the torque controlled first drive device is gradually increased till the current of the second drive device equals the current of this motor in its de-coupled state. Then, the current of the first drive device is measured and the difference is calculated between this current and the current of the first drive device in its de-coupled state. Finally, a new set point current is introduced for the first drive device being the current of this motor in de-coupled state raised with 50% of said difference. By doing so the current of the second drive device, which is speed controlled, is allowed to increase till a stationary value is reached. The losses in the transfer zone are and will remain equally shared over the respective drive devices.
The first image-carrying member can be an image-forming member or an intermediate transfer member, while the second image carrying member can be an intermediate transfer member or a receptor material. Examples of image-forming members are drums or belts with a photoreceptive or a magneto-sensitive outer layer. Examples of intermediate transfer members are seamed or seamless intermediate transfer belts. Such an intermediate transfer belt may be composed of an electrically semi-insulating or insulating material with a low surface energy, or comprises at least a top coating of such a material. Examples of such a material are polyesters such as e.g. Hytrel 7246, polyimides, polycarbonates or dissipative polymer blends. A plurality of intermediate members, being drums or belts, can be used. The intermediate transfer member in contact with the receptor material is preferably a belt. More preferably, the intermediate transfer member in contact with the receptor material is a belt being at least locally heated prior to contacting the receptor material to simultaneously transfer and fuse the image to the receptor material. This belt may comprise an electrically conductive backing member, such as a metal, covered for example with a silicone elastomer, polytetrafluoroethylene, fluorosilicones, polyfluoralkylene and other fluorinated polymers. Optionally, on top, a semi-insulating or insulating coating layer of, for example, a fluorosilicone, may be formed. Alternatively, a fabric backing may be used covered with a conductive (conformable) silicone layer, optionally covered with a top coating. In case a fabric backing is used, a pre-stressed fabric backing or a reinforced fabric backing is preferably used to increase the belt stiffness.
The receptor material can be in web form or in sheet form. In the latter case, the receptor material is preferably transported on a conveyor. Typical materials are paper, films, label stock, cardboard etc.
In another aspect of the invention, an image reproduction system is disclosed which includes a device for transferring developed images from an image-carrying member to one face of a receptor material, comprising
a transfer member capable of being coupled with, and de-coupled from, both said image-carrying member and said receptor material, wherein said transfer member and said receptor material pass over respective guide rollers so positioned, in the coupled position of said transfer member with said receptor material, to form a transfer contact zone therebetween;
a first drive device for tensioning said receptor material; and
a second and a third drive device, both capable of speed and torque control, for driving said transfer member and said receptor material respectively such that the additional load created in said transfer contact zone is balanced between said second and third drive device.
To enable the simultaneous transfer and fusing of a developed image on the receptor material, the transfer member can be at least locally heated prior to the transfer contact zone. By arranging for the heated transfer member to be de-coupled from the receptor material, at shut-down, the risk of overheating the receptor material and possibly causing a fire hazard is reduced.
The image-carrying member can be an image-forming member such as e.g. a photosensitive drum. Dependent on the application envisaged, whether it concerns a monochrome or a multi-colour reproduction system, a single pass or a multi-pass system, in operation, a single or a plurality of image-forming members, each of a separate colour, may contact the image-carrying member in a first transfer contact zone or in a plurality of first transfer contact zones, when appropriate. The image-forming member(s) may be de-coupable and separately driven by a drive device capable of speed and torque control. If so, the losses due to the contacting in the first transfer contact zone(s) can be balanced between the drive device of the image-carrying member and the drive device of the respective image-forming member. Alternately, the image-forming member(s) can be driven by adherent contact with the image-carrying member, see e.g. U.S. Pat. No. 5,808,967 (De Bock et al., Xeikon N.V.). In this case, no separate drive devices are provided for driving the image forming member(s).
The image-carrying member can be an intermediate transfer member. A separate drive device, capable of speed and torque control is provided to independently drive the intermediate transfer member. In a preferred construction, the intermediate transfer member and the transfer member pass over respective guide rollers so positioned, in the coupled position of the intermediate transfer member with the transfer member, to form an intermediate transfer contact zone therebetween. At least one of these guide rollers may be movable to enable the intermediate transfer member and the transfer member to be de-coupled from each other. The additional load created in said second transfer contact zone is balanced between the drive devices of the intermediate transfer member and the transfer member respectively.
One or both of the image-carrying member and the transfer member are preferably in the form of members having a continuous surface, in particular in the form of endless belts. In the following general description, where reference is made to belts, it is to be understood that a belt could be replaced by another member having a continuous surface, such as a drum, where the context so allows.
Besides the benefits already mentioned, including slipless drive, the present invention has a number of additional advantages. At start-up, the transfer belt, the image-carrying belt and the receptor material can be brought up to speed before coupling, reducing the shock to delicate components of the printer. Distortion when the reproduction system remains idle for a significant period of time is also avoided.
An added advantage of being able to run the image-carrying belt at a controlled speed in the de-coupled state independent of the transfer belt and the receptor material, is that calibration of the printing process can be undertaken with the image-carrying belt running at a reduced speed, enabling a higher level of toner to be deposited enabling the calibration to be made more accurately.
The following is of particular interest for receptor materials in web form. By enabling the web to be brought up to speed in the de-coupled state after the image-carrying belt and the transfer belt are already running, indeed even after the transfer belt and the image-carrying belt are coupled with each other, reduces the loss of receptor material which otherwise occurs at start-up of the printer. The web can be brought up to speed and coupled with the already moving transfer belt once the latter has reached its operating temperature and just as the first image to be transferred is approaching. The issue involved is an issue of synchronization. This is handled in the co-pending patent application EP1079283 (Xeikon N.V.).
The speeds of the image-carrying belt, the transfer belt and the tensioned receptor material are adjusted while these respective members are de-coupled such that they will all be moving at about the same speeds, within predetermined thresholds. Subsequently all these respective members are coupled such the losses of the respective transfer contact zones are balanced over the respective drive motors. Particularly, the coupling may be such that the only direct controlled drive device left, is the second drive device which drives the receptor material. The receptor material is set to a predetermined speed and acts as the xe2x80x9cmasterxe2x80x9d, while the other image-carrying members act as xe2x80x9cslavesxe2x80x9d. To accomplish this, first the intermediate transfer member is slaved to the transfer member such that the losses in the intermediate transfer contact zone are balanced over the respective drive device. Subsequently, the transfer member, which is already coupled to the intermediate transfer member, is slaved to the receptor material such that by controlling the drive of the receptor material, the entire system is controlled. This balanced configuration not only minimizes the effect of image distortion in each transfer contact zone, but assures also that the image distortion remains substantially unchanged over time. This control over the image distortion on component level is quite important because, although it is nearly impossible to correct for a fluctuating overall image distortion adequately, it is rather straightforward to adjust for a constant and limited overall image distortion on a system level by control of the writing speed on the image-forming member(s).
To achieve speed adjustments in de-coupled and/or coupled state, the reproduction system may further include devices for measuring the speeds respectively of the image-carrying belt, the transfer belt and the receptor material and a control device for adjusting the power fed to the drive devices. The device for measuring the speed of the image-carrying belt may include an encoder driven by the image-carrying belt. This arrangement is preferred over the positioning of an encoder on the associated drive device. Similarly, the device for measuring the speed of the receptor material may include an encoder driven by the receptor material. The device for measuring the speed of the transfer belt may include a device for detecting the passage of one or more timing marks on the transfer belt past a predetermined location.
The drive devices are preferably in the form of independently controllable drive motors. The drive devices are preferably selected from electric motors. In a preferred embodiment, at least the two slave drive motors, and preferably also the master drive motor, are each constituted by a DC drive motor controllable between at least two operating modes, namely a constant speed mode and a constant torque mode. Such motors operate in such a manner that the application of a constant voltage corresponds to the constant speed mode while the application of a constant current corresponds to the constant torque mode.
The reproduction system may be adapted for duplex reproduction, by further including a further image-carrying member, a further transfer member capable of being coupled with the further image-carrying member and the receptor material to transfer images from the further image-carrying member to the opposite face of the receptor material, the further image-carrying member and the further transfer member having respective controllable further drive motors associated therewith.
Duplex printing may be then achieved by driving the further image-carrying member, and the further transfer member while the further transfer member is de-coupled from the further image-carrying member and the receptor material, and thereafter coupling the further transfer member with the further image-carrying member and the receptor material such that the losses created in the respective transfer contact zones are shared over the respective members.